1. Field of the Invention
The present invention relates generally to spatial light modulators. More particularly, the present invention relates to improved resolution in microelectromechanical optical devices.
2. Related Art
Spatial light modulators (SLM) have found use in a variety of applications, including their use in image displays. Of particular interest are SLM manufactured using microelectromechanical systems technology (MEMS), such as a grating light valve (GLV) or digital mirror device (DMD). Operation of MEMS optical devices is similar, relying on mechanical deflection of microscopic optical structures fabricated on the device to reflect or diffract impinging light.
For example, a grating light valve (GLV) can be used to modulate light intensity to implement a display as disclosed in U.S. Pat. No. 6,215,579 issued to Bloom et. al. The GLV is used to modulate light intensity by electrostatic deflection of long thin microscopic optical structures (“ribbons”) to create a diffraction grating. The electrostatic deflection is accomplished by applying a control voltage to the ribbon. Typically, half the ribbons remain in a fixed position, and the other half are deflected by distances of less than one quarter of a wavelength of the incident light by applying a voltage to the ribbons. The more the deflection, the deeper the diffraction grating, and hence the more light is diffracted.
A two dimensional display may be produced by reflecting a beam of light from the GLV and sweeping the beam across the display. To create a pixel, a voltage proportional to the desired pixel value is applied to half the ribbons corresponding to the pixel (while the other half of the ribbons are fixed in position). A vertical column of pixels is generated by the GLV, and the pixel intensity is modulated as the beam is swept across the display horizontally to produce a two dimensional array of pixels. Each pixel is thus defined by GLV ribbons in the vertical dimension, and by the pixel time in the horizontal dimension. The pixel time and horizontal scan rate determine the horizontal pixel-width of the display. Alternatively, the GLV may be used to produce a row of pixels which is modulated as it is swept across the display vertically. For purposes of this discussion, it will be assumed that horizontal scanning is used for convenience of illustration and should not be considered limiting.
The vertical resolution of a display produced by a GLV is determined by the number of ribbons and how they are combined to produce pixels. For example, Bloom discloses the use of 1920 ribbons, configured 6 per pixel to produce a 320-pixel display. A minimum of two ribbons per pixel is typically required, since the diffraction grating is produced by alternating fixed ribbons with deflecting ribbons. Fixed (“reference”) ribbons are tied to a bias voltage (typically ground), and deflecting (“active”) ribbons are deflected by the application of a ribbon control voltage. As noted by Bloom, different assignment of ribbons to pixels is possible, e.g. using 2, 4, 8, 10, or 12 ribbons per pixel. This assignment is defined by the electrical interconnection on the integrated circuit substrate, and is fixed at manufacturing time.
Maximum resolution of a GLV can be obtained by connecting each ribbon pair to a separate interconnect pin. Such an approach is impractical for a high-resolution display, however, because a large number of interconnects would be required. Practical packages are limited to 200-300 pins, far less than the 3000 or so ribbons typically provided by a GLV. Furthermore, a significant cost component of a packaged GLV is the many bond wires that are required to connect the GLV ribbons to the package pins.
Operation of a GLV can be in either a linear (analog) or non-linear (digital) mode. The non-linear (digital) mode of operation disclosed in U.S. Pat. No. 5,311,360 issued to Bloom et. al. makes use of a hysteresis effect that causes ribbons to latch in a down position when a sufficiently high ribbon control voltage is applied to the ribbon. Although operation in this mode provides some advantages in low power consumption and simplified interface, it limits the ability to provide gray scale control of intensity. To provide gray scale operation, a binary encoding scheme is disclosed in U.S. Pat. No. 5,677,783 issued to Bloom et. al. which uses 30 ribbons, grouped as 1 pair, 2 pairs, 4 pairs, and 8 pairs where each group is controlled separately to provide 4-bit (16-level) gray scale control. This scheme, however, suffers from several limitations; the large number of ribbons per pixel required results in low resolution, and the trade-off between gray-scale resolution and pixel resolution is fixed at manufacturing time.
The linear (analog) mode of operation disclosed in U.S. Pat. No. 6,215,579 limits the amount of deflection of the ribbons to a small amount, such that the deflection is roughly proportional to the applied voltage. This approach allows direct control of gray-scale values by applying an analog voltage directly to the groups of ribbons forming a pixel, but still suffers from the limitation that the assignment of ribbons to form a pixel must be fixed at manufacturing time.
A row-column addressing scheme to reduce the number of interconnects required in a large pixel display is disclosed in U.S. Pat. No. 5,841,579, issued to Bloom et. al. The row-column addressing scheme disclosed, however, is only applicable to a GLV operated in the non-linear (digital) mode since it relies on the hysteresis property that the ribbon will snap to the fully deflected position if a voltage exceeding a threshold is applied. In the row-column addressing scheme, half the required threshold voltage is applied to the row and half to the column corresponding to an addressed pixel. Only the addressed pixel will have the full voltage applied (and snap to the deflected position); all other pixels in the row and column will deflect only slightly. This slight deflection of the non-addressed pixels can result in some reduction in the contrast of the display, as noted by Bloom. Unfortunately, such a row-column addressing scheme is difficult in a GLV operated in a linear (analog) mode. In the linear mode, the ribbon deflection is proportional to the applied voltage, and the row-column addressing scheme would result in unacceptable crosstalk between pixels in the same row or column.
Providing sub-pixel resolution in displays has not heretofore been possible. Sub-pixel resolution can be simulated in displays using the technique disclosed in U.S. Pat. No. 4,720,705 issued to Gupta et. al. where adjacent pixel gray-scale values are altered to simulate sub-pixel placement of edges. Although this technique can improve the apparent resolution for some applications (e.g. text display), it is inappropriate for other applications that require bright objects to be placed precisely (e.g. lights in a simulator).
Finally, when projecting images onto non-planer surfaces, image distortion occurs. Correction of this distortion can be implemented without complex optical lenses by non-linear image mapping, e.g. by electronically adjusting the displayed pixels to compensate for the distortion as disclosed in U.S. Pat. No. 5,850,225 issued to Cosman. Electronic compensation approaches suffer from significant complexity due the intense processing required.